The gear shift schedule for the automatic transmission of a vehicle is conventionally determined by two inputs: the engine power level and the transmission output speed. The shift schedule is executed by a control logic system located within the transmission. A governor unit, also within the transmission, provides the input to the control logic system based upon transmission output speed, while the input as to the engine power level is provided by the disposition of the engine throttle, which is linked to the transmission throttle valve. A typical prior known transmission throttle valve linkage is diagrammatically illustrated in FIG. 1A of the drawings wherein the vehicle engine (E) drives a transmission (T) and the throttle control of the engine is accomplished by linkage interconnecting an operator's accelerator pedal (OP) to the transmission throttle valve (TV) through the engine throttle (ET).
In a more advanced vehicular power train control system, an electric motor (M) is used for driving the engine throttle (ET), and a representative implementation of this prior known arrangement is illustrated in FIG. 1B of the drawings. In this embodiment the electric motor (M) drives the engine throttle (ET) based upon the operator's accelerator pedal position (OP) and other "condition inputs" (CI) fed into a computer processing unit. This motor controlled engine throttle is effective in vehicular traction control, because the engine throttle opening can be reduced in response to condition inputs (CI) in order to eliminate excessive wheel spin, as, for example, in the situation when an operator attempts to effect rapid vehicular acceleration on a slippery road surface.
One might reasonably expect that for an ideal coordination of the vehicle engine with its transmission, and particularly with a hydraulically controlled transmission, the transmission throttle valve (TV) should be connected to the engine throttle (ET), as illustrated in FIG. 1B. However, various difficulties are encountered with this arrangement. For example, the motion of the throttle valve (TV) plunger is determined by the balance of three forces: (1) F.sub.S, which is the throttle valve spring force; (2) F.sub.C, which is the engine throttle motor (M) force through the transmission (TV) linkage; and, (3) F.sub.P, which is the hydraulic pressure force of the throttle valve. For the throttle valve plunger to advance, it is required that: EQU F.sub.C &gt;F.sub.S -F.sub.P.
The motor (M), which actuates the engine throttle (ET), in turn, needs to generate a force F.sub.M which is sufficient to drive both the engine throttle (ET) and the transmission throttle valve (TV) linkage, and this relationship may be stated by the mathematical expression: EQU F.sub.M =F.sub.C +F.sub.T +rotor inertial reaction
In the above relationship, F.sub.T is the force required to drive the engine throttle (ET) which is usually counter-rotated by a shut-off spring to return the throttle to a closed position when the engine throttle motor (M) is not being actuated to open the engine throttle (ET). Finally, if the transmission throttle valve linkage is arranged as illustrated in FIG. 1B of the drawings, then the motor (M) must be of such a size as to deliver a force F.sub.M wherein: EQU F.sub.M &gt;F.sub.R +F.sub.T +F.sub.S -F.sub.P
In the above relationship F.sub.R is the maximum inertial reaction force for maximum acceleration of the rotor in motor (M), and the condition of the hydraulic throttle valve pressure F.sub.P in the transmission throttle valve (TV) becomes an important consideration.
Conventionally, all hydraulic pressures within a hydraulically operated automatic transmission for vehicles are generated by a hydraulic pump which is driven directly by the vehicle engine (E). Accordingly, before the engine can start driving the pump, the transmission throttle valve pressure F.sub.P must be zero. Therefore, in order for the engine throttle motor (M) to advance the engine throttle (ET) for starting, the motor (M) must deliver a force that may be stated by the following mathematical expression: EQU F.sub.M &gt;F.sub.R +F.sub.T +F.sub.S
The aforesaid relationship will require a much large engine throttle motor (M) than needed for running the power train under normal conditions. In practice, and to save the cost of a larger engine throttle motor (M), the performance is compromised by attaching the transmission throttle valve (TV) linkage to the vehicle accelerator pedal (OP) as illustrated in FIG. 1C of the drawings. In this configuration, a smaller engine throttle motor (M) may be used but by doing so it has been found that the vehicle transmission shift schedule will not be correct under those conditions where wheel anti-spin is desired. In such a condition, the vehicle engine speed may tend to increase, when, to the contrary, it should be reduced by at least partially retarding the engine throttle setting.